The present invention generally relates to optical disk recording media, and more particularly, to high density optical discs for use with an optical recording/reproducing apparatus for optically recording or reproducing information.
A conventional optical apparatus will be described with reference to FIG. 17. The irradiation beams of a semiconductor laser 81 are converted into 0 order and primary diffraction beams by a diffraction grid 82, thereafter are reflected by a beam splitter 83, are focused onto a recording layer 80 of an optical disk 85 by an objective lens 84 so as to form an optical spot 86. Optical beams 88 reflected by the optical disk 85 are transmitted through the objective lens 84, and are split by splitter 83 so that the transmitted optical beams 88 enter an optical detector 87. A focus error signal together with a reproducing signal is detected by an astigmation method from an optical detector 87a having four sections. A tracking error signal is obtained by a three-beam method of detecting the optical quantity difference of the previous .+-. primary diffraction returning beams which become incident to the optical detectors 87b and 87c. The reproducing signal is obtained as a total of the beam reception amount of the optical detector 87a. In such a conventional optical disk apparatus, the recording layer 80 is the only layer on the disk face. The recording capacity increases if the recording layers 80 are laminated in a direction of the thickness of the disk 85. When the recording layers 80 have been laminated, the influences of interfering beams from the recording layers which are not being recorded or reproduced become a problem, thus resulting in difficulties in the lamination of multiple recording layers unless the problem is solved.
In recent years, higher density recording, using shorter wave length beams, narrower tracks or the like have been tried. Furthermore, the recording density can be improved if the density improvement is effected in the vertical direction of the recording face in addition to the improvement within such a recording face, namely, if the number of faces can be increased by the lamination of the recording layers. Such a lamination has not been adopted because of the disadvantage in that there are influences of the reflection and transmission of the beams caused by a layer other than the reproducing/recording layer. Even in the construction of the conventional optical disk apparatus, the number of recording faces provided on the optical disk medium is limited to only one face on each side. The number of recording faces provided with respect to one optical disk medium is therefore two faces at most.